Cellulosic fibers are a basic component of absorbent products such as diapers. These fibers form a liquid absorbent structure, a key functioning element in the absorbent product. Cellulosic fluff pulp, a form of cellulosic fibers, is a preferred fiber for this application because a high void volume or high bulk, liquid absorbent fiber structure is formed. This structure, however, tends to collapse on wetting. The collapse or reduction in fiber structure bulk reduces the volume of liquid which can be retained in the wetted structure and inhibits the wicking of liquid into the unwetted portion of the cellulose fiber structure. Consequently, the potential capacity of the dry high bulk fiber structure is never realized and it is the fiber structure's wet bulk which determines the liquid holding capacity of the overall fiber structure.
Fiber structures formed from crosslinked cellulosic fibers generally have enhanced wet bulk compared to those formed from uncrosslinked fibers. The enhanced bulk is a consequence of the stiffness, twist, and curl imparted to the fiber as a result of crosslinking. Accordingly, crosslinked fibers are advantageously incorporated into absorbent products to enhance their wet bulk.
Polycarboxylic acids have been used to crosslink cellulosic fibers. See, for example, U.S. Pat. No. 5,137,537; U.S. Pat. No. 5,183,707; and U.S. Pat. No. 5,190,563. These references describe absorbent structures containing individualized cellulosic fibers crosslinked with a C2-C9 polycarboxylic acid. Absorbent structures made from these individualized, crosslinked fibers exhibit increased dry and wet resilience and have improved responsiveness to wetting relative to structures containing uncrosslinked fibers. Furthermore, a preferred polycarboxylic crosslinking agent, citric acid, is available in large quantities at relatively low prices making it commercially competitive with formaldehyde and formaldehyde addition products.
Despite the advantages that polycarboxylic acid crosslinking agents provide, cellulosic fibers crosslinked with low molecular weight polycarboxylic acids such as citric acid, tend to lose their crosslinks over time and revert to uncrosslinked fibers. For example, citric acid crosslinked fibers show a considerable loss of crosslinks on storage. Such a reversion of crosslinking generally defeats the purpose of fiber crosslinking, which is to increase the fiber's bulk and capacity. Thus, the useful shelf-life of fibers crosslinked with these polycarboxylic acids is relatively short and renders the fibers somewhat limited in their utility. Polymeric polycarboxylic acid crosslinked fibers, however, exhibit a density that remains substantially unchanged over the life-time of fibrous webs prepared from these fibers. See, for example, U.S. Pat. No. 6,620,865. This resistance to aging or reversion of density relates to the stable intrafiber crosslinks formed using polymeric polycarboxylic acid crosslinking agents. In contrast, cellulose fibers crosslinked with citric acid show a considerable increase in density, accompanied by a loss of bulk and absorbent capacity over time. Generally, the increase in density indicates a decrease in the level of crosslinking (i.e., reversion) in the fibers. In addition to density increase, the loss of crosslinking in the fibrous web results in a less bulky web and, consequently, diminished absorbent capacity and liquid acquisition capability.
Unfortunately, citric acid or polycarboxylic acid crosslinking agents can cause discoloration (i.e., yellowing) of the white cellulosic fibers at the elevated temperatures required to effect the crosslinking reaction.
Bleaching is a common method for increasing pulp brightness of pulp. Industry practice for improving appearance of fluff pulp is to bleach the pulp to ever-higher levels of brightness (the Technical Association of the Pulp & Paper Industry (“TAPPI”) or the International Organization for Standardization (“ISO”)). Traditional bleaching agents include elemental chlorine, chlorine dioxide, and hypochlorites. However, bleaching is expensive, environmentally harsh, and often a source of manufacturing bottleneck. Widespread consumer preference for a brighter, whiter pulp drives manufacturers to pursue ever more aggressive bleaching strategies. While highly bleached pulps are “whiter” than their less-bleached cousins, these pulps are still yellow-white in color. A yellow-white product is undesirable. Countless studies suggest that consumers clearly favor a blue-white over a yellow-white color. The former is perceived to be whiter, i.e., “fresh”, “new” and “clean”, while the latter is judged to be “old”, “faded”, and “dirty”.
In addition to fiber discoloration, unpleasant odors can also be associated with the use of α-hydroxy carboxylic acids such as citric acid. Recently, it was found that the characteristic odor associated with citric acid crosslinked cellulosic fibers could be removed and the brightness improved by contacting the fibers with an alkaline solution (e.g., an aqueous solution of sodium hydroxide) and an oxidizing bleaching agent (e.g., hydrogen peroxide). See U.S. Pat. No. 5,562,740. In the method, the alkaline solution raises the finished fiber pH preferably to the 5.5-6.5 range from about 4.5. This, in combination with the oxidizing bleaching agent, eliminates the “smokey and burnt”odor characteristics of the citric acid crosslinked fibers. The oxidizing bleaching agent also helps to increase final product brightness.
Accordingly, there exists a need for crosslinked cellulosic fibers having advantageous bulk and improved brightness and whiteness. The present invention seeks to fulfill these needs and provides further related advantages.